1. Field of the Invention
The field of the present invention is methods for withdrawing a workpiece from a drawing mold by a cross bar having holding means such as a suction cup to transport the workpiece drawn by the drawing mold toward a mold of a next step.
2. Description of the Prior Art
There is a conventionally known so-called transfer press comprising a plurality of molds such as a drawing mold, a trimming and piercing mold and a bending mold which are arranged in line in the order of processing, and a pair of left and right feed bars mounted on opposite sides of the molds for cyclic motion in a vertical plane and connected together by a plurality of cross bars, wherein a workpiece adsorbed or attracted by a holding means such as a suction cup mounted on the cross bar is transported sequentially toward the mold of the next step by the cyclic motion.
The cyclic motion of the feed bar in such transfer press is generally formed of a combination of an upward movement, a horizontal movement, a downward movement and the like. For this reason, a large inertia force acts on the feed bar and/or the cross bar when the motion changes from a vertical movement to a horizontal movement and vice versa, providing a cause for vibration in the cross bar which has a relatively low strength. If the cross bar is formed to have a larger thickness to increase its rigidity in order to prevent this disadvantage, the inertia force may be increased by an increase in weight to bring about inconveniences such as making vibration further violent and necessitating a larger-sized drive mechanism.
In order to avoid the above disadvantages, it can be conceived that a path for the cyclic motion of the feed bar which has hithereto been a combination of the vertical movement and the horizontal movement is formed of a smooth curved line, thereby reducing the generation of the inertia force.
However, among the plurality of molds used in the transfer press, the drawing mold includes a blank holder liftably disposed around the outer periphery of a lower die, unlike the other molds, and the blank holder is adapted to be raised to an upper position above the lower die along with the workpiece after completion of the drawing. For this reason, if the cross bar on standby in an intermediate position between the drawing mold and the mold of the next step is intended to be moved to a position between the lower and upper dies of the drawing mold through a movement path formed of the above-described curved line, there is a possibility of the cross bar interfering with the raised blank holder. If the cross bar holding a workpiece is likewise intended to be moved from the drawing mold toward the mold of the next step along the movement path in the form of the curved line, there is a possibility that the cross bar or the workpiece held by the cross bar interferes with the blank holder. For such a reason, there is a need for independently establishing a path for movement of the cross bar along which the bar withdraws a workpiece from the drawing mold and a path for movement of the cross bar along which it withdraws a workpiece from the other mold. This may be a cause for complication of the structure of a transporting mechanism and lowering in the transporting speed.